By Eleanor Williams BBC News, Dorset

Kate and the officers leave Weymouth police station

Underage drinking is a problem in most parts of the country and Dorset is no exception.

With the festive season in full swing, police and trading standards officers are determined not to let underage alcohol sales spiral out of control.

They are carrying out a number of undercover stings on off-licences, corner shops and supermarkets across the county using a teenager as bait to see if any of them will sell alcohol.

Two trading standards officers, Chris and Tim, and police licensing officer Karen O'Donnell are all in plain clothes when they meet for the briefing at Weymouth Police station.

Fifteen-year-old Kate is also there. She is one of trading standards most experienced volunteers as she has been doing undercover work for about a year.

She says she really enjoys it and sometimes jokingly calls herself a "secret agent".

Chris waits on the corner near the shop being targeted

Before Wednesday night's operations can get under way she is photographed and measured to make sure she "looks like your typical teenager".

Ivan Hancock, divisional trading standards manager, is accompanying the team on the sting and explains: "This will help if the case has to go to court and means the volunteer won't have to be called as a witness."

"It's important that they don't look older than they are, it needs to be realistic."

The first target is a supermarket on the outskirts of Weymouth. Kate walks in on her own while the officers keep watch, pretending to look through magazines or buying a bar of chocolate.

But they have to make sure they stay close to her to overhear any conversation with the sales assistant.

This one is a sale straight away - no questions asked.

Chris talks to staff at one of the shops that refused a sale

Kate walks out with four cans of lager and the team gathers by the car to seal the evidence in a clear bag.

Kate comes across as a typical teenager, giggles a lot and says she gets a bit nervous before she walks into the shop.

"I do get really nervous, especially in the beginning, but I'm glad we got a sale in the first supermarket because they [supermarkets] are my worst."

All the details of the sale and the name of the sales assistant are logged and the officers discuss what action is to be taken.

Karen decides to return the next day to issue a fixed penalty of �80.

Under new laws that came into force a year ago police can issue on-the-spot fines in those cases where it is felt the staff has had adequate training but still went on to sell alcohol to the teenagers.

"If that's the case it will be the sales assistant who receives the fine," she says.

"There are sometimes when you just think 'please, please don't sell cause I don't want to have to do this to this person'."

All details have to be recorded in case they are needed as evidence

Trading standards officer Chris Legg, who has been on a number of undercover stings in the past, admits he sometimes feel "guilty".

"We've had some shops in the past where staff have been sacked because they've sold and you start to feel a bit guilty, but then you think if a 15-year-old gets alcohol and ends up in the road and gets killed."

The team then moves on to the next targets, a couple of off-licences, and two corner shops, but there are no more sales on this night.

The last shop where Kate attempts to buy a bottle of cider is Londis on Abbotsbury Road, but she is turned down.

Julie Haazen, store supervisor, is surprised when she finds out.

"I had no idea," she says.

"I think it's a good thing to be checked. Obviously mistakes do happen but we are quite strict here."

She said they do get a lot of youngsters trying to buy alcohol and "they come up with all kinds of excuses".